The present invention relates to a new and improved machine tool having a chuck which grips a workpiece during a machining operation.
Many known machine tools have chucks which are actuated by hydraulic motors to grip workpieces. The time required to actuate the chuck to grip a workpiece will vary as a function of the size of the workpiece and the fluid pressure used to operate a chuck actuator motor. Due to the different time periods required to actuate the chuck under different operating conditions, the machine tool controls have previously been programmed to provide sufficient time to close the chuck under the most adverse circumstances, that is when chuck jaw travel is a maximum and chuck actuator motor pressure a minimum. Of course, this results in the providing of an excessive amount of time for chuck actuation under conditions which require less time.
During the machining of a workpiece, there may be rough and finish cutting operations. During a rough cutting operation, relatively large forces are applied to the workpiece by a cutting tool so that a chuck must grip the workpiece with relatively large jaw forces. When a finish cutting operation is to be undertaken, the operating forces on the workpiece will be less and therefore the chuck jaw forces can be reduced.
Reducing the chuck jaw forces before undertaking a finish cutting operation tends to reduce the extent to which the workpiece is deformed by the chuck. For example, when thin tubular stock is to be machined, the relatively large chuck jaw forces used during a rough cutting operation may elastically deform the workpiece. If these chuck jaw forces are maintained during the finish cutting operation, the workpiece tends to spring back when released by the chuck jaws. This decreases the accuracy with which the workpiece is machined.